The present invention relates to a gas discharge closing switch and, more particularly, to a hydrogen thyratron having an annular keep-alive structure.
Gas discharge closing switches, such as thyratrons, are used for rapid switching of high power signals with low power consumption. A typical thyratron has an anode connected to high voltage and a cathode held at ground potential. A control electrode or "grid" is placed between the anode and the cathode which, upon application of a positive control pulse, closes the switch by drawing electrons from the cathode to transform gas within the device into a dense, conducting plasma in an avalanche process.
In order to facilitate breakdown of the gas into a conductive plasma, it is desirable to provide a localized pool of charge carriers within the device on a continuous basis. This is done by pre-ionizing a portion of the gas within the device, typically using a low power DC discharge between an auxiliary "keep-alive" electrode and the cathode itself. The resulting pool of carriers, made up of both ions and electrons, is referred to as a "keep-alive plasma". It dramatically improves (decreases) the rise time of the cathode current and thereby prolongs the life of the device.
One prior form of auxiliary electrode is a wire ring located just inside the side wall of a cathode heat shield. Such electrodes are rather inefficient, however, because the keep-alive plasma is not in line with the field from the control electrode and therefore is not located in the region of ultimate gas breakdown, i.e., any region in the direct path of current flow during conduction. Consequently, the charged particles of the keep-alive plasma are less readily available to perform their function.
Another auxiliary electrode is a solid disk disposed within an aperture at the end of a cathode heat shield. This structure, disclosed in U.S. Pat. No. 4,123,684 to Menown et al., provides a localized keep-alive plasma in the vicinity of the electrode. It suffers, however, from the fact that all of the current through the device must pass through a relatively small space between the disk and the heat shield, which may limit the maximum peak current capability of the thyratron.
Yet another auxiliary electrode structure is disclosed at page II-16 of Research Study on Hydrogen Thyratrons-Vol. II (1956) (authors unknown). The auxiliary electrode is shown near a control electrode, remote from the cathode and its heat shield, for generating a "triggering plasma". Although the structure of the electrode is not entirely clear from the publication, it is apparently used to generate a keep-alive plasma over a relatively large region between the control electrode and the cathode. Unfortunately, this arrangement generates a rather low density plasma, providing relatively few carriers to facilitate breakdown when the control electrode is pulsed. In addition, a large negative bias must be applied to the control electrode to prevent the keep-alive plasma from causing premature breakdown in the anode region. This arises from the fact that the charged particles of the keep-alive plasma are not localized to the region where they are needed.
Therefore, it is desirable in many applications to provide a structure for maintaining a keep-alive plasma localized to the area of ultimate breakdown without restricting the flow of current after breakdown.